Vehicle-to-Home Backup Savings Estimator

Core Cost Formula

All three backup options are evaluated using the same basic cost-per-kWh framework. The calculator computes a levelized cost of backup energy:

• c: levelized cost of backup energy ($/kWh)
• A: annualized capital cost for the backup hardware ($/year)
• O: annual operating expense related to outages ($/year)
• E: backup energy delivered during outages (kWh/year)

By keeping the formula consistent, you can make an apples-to-apples comparison between using your EV, a separate battery, or a generator. The differences come from how each option incurs capital costs, operating costs, and energy losses.

Modeling the Three Backup Options

1. EV Vehicle-to-Home Backup

The EV backup scenario assumes the vehicle battery already exists for transportation, so its full purchase cost is not attributed to backup. Instead, the model focuses on:

• Effective backup capacity: EV battery capacity (kWh) multiplied by the percentage you are willing to reserve for backup and by the bidirectional efficiency.
• Energy demand during outages: expected outage hours per year times the average home load during an outage.
• Operating cost: the cost of recharging the EV battery from the grid after an outage, based on your retail electricity price and the round-trip efficiency.
• Capital cost: the incremental cost of bidirectional charging hardware, approximated as a share of a comparable stationary battery system and annualized using your discount rate.

In this framework, the EV becomes a resilience asset because it can supply a certain number of kWh each year during outages. The more frequently it is used for backup (and the more outage hours you experience), the more those fixed hardware costs are spread out over delivered energy.

2. Standalone Battery System

The standalone battery scenario assumes you purchase a dedicated battery and inverter sized to support your outage load. Key modeling elements include:

• Usable capacity: the entered battery usable capacity in kWh, which may be less than the nameplate rating to reflect depth-of-discharge limits.
• Cycle life: the total number of full equivalent cycles you expect over the battery’s lifetime.
• Capital recovery: the up-front battery system cost is amortized over its expected service life using your discount rate, converting a one-time purchase into an equivalent annual cost.
• Operating costs: these are usually modest for batteries and mainly consist of charging energy from the grid, adjusted for round-trip efficiency if modeled in the script.

The battery’s cycle life effectively caps how much backup energy it can provide over its life. Capital cost per kWh is driven by both the purchase price and how many times you intend to cycle the battery in outage situations.

3. Fuel-Powered Generator

The generator scenario treats the generator as a fuel-constrained device with relatively low upfront cost but higher variable costs. The model focuses on:

• Fuel cost per delivered kWh: your input cost already accounts for generator efficiency and any delivery or storage overhead, expressed per usable kWh output.
• Annual maintenance: fixed yearly spending on oil changes, tune-ups, inspections, and other upkeep required to keep the generator reliable.
• Energy demand during outages: the same outage hours and average load inputs, assuming the generator can meet that load.

For generators, the operating cost term O is usually dominant, especially where fuel is expensive or outages are long. Capital costs can be important for whole-home standby units but are not always the main driver of cost per kWh.

Interpreting the Results

When you run the calculator, you will typically see a cost per backup kWh for each option, along with intermediate values such as usable energy or implied annual capital cost. You can use these outputs to answer questions such as:

• Is leveraging my existing EV and a bidirectional charger cheaper than buying a new home battery?
• At what fuel price does a generator become more expensive than a battery or EV backup?
• How sensitive are my results to outage hours per year or the percentage of my EV battery I am willing to allocate?

Generally, an EV-based solution becomes more attractive when:

• You already own (or plan to own) an EV with a relatively large battery.
• You are comfortable reserving a meaningful share of that battery for backup without compromising mobility.
• Outages are infrequent enough that battery wear from backup use is modest.
• Electricity to recharge the EV is cheaper than generator fuel on a per-kWh basis.

A dedicated battery can make more sense when you want fully automatic coverage, do not want to rely on your car being at home, or want to avoid additional cycling on your EV battery. Generators tend to be favored where fuel is inexpensive, power needs are very high, or outages are rare but occasionally long.

Worked Example

Consider a household with the following baseline values (similar to the defaults in the form):

• EV battery capacity: 77 kWh
• Usable for backup: 70%
• Bidirectional efficiency: 88%
• Average home load during outage: 4.5 kW
• Expected outage hours per year: 36 hours
• Retail electricity price: $0.17/kWh
• Generator fuel cost: $0.42/kWh delivered
• Generator maintenance: $220/year
• Standalone battery system cost: $11,000
• Standalone battery usable capacity: 13.5 kWh
• Standalone battery cycle life: 4,000 cycles
• Discount rate: 5%

The annual backup energy requirement is about 36 hours × 4.5 kW = 162 kWh per year. The EV can provide roughly 77 × 70% × 88% ≈ 47 kWh in a single discharge. That is enough to cover shorter outages entirely, or to partially cover longer events depending on your strategy and how often you recharge.

In the EV case, the main recurring cost is the electricity needed to recharge the 47 kWh used for backup. At $0.17/kWh, that is about $8 per full backup event, before considering any round-trip losses. The capital cost of the bidirectional charger is spread over many years and many outage events, so on a per-kWh basis it may be modest, particularly if you use the hardware for other services such as demand response.

For the standalone battery, the $11,000 capital cost is spread across its expected lifetime energy throughput. If 4,000 cycles at 13.5 kWh each are fully utilized, that is 54,000 kWh over the life of the system. Ignoring discounting for illustration, the raw capital component is roughly $11,000 ÷ 54,000 ≈ $0.20/kWh of lifetime throughput, before adding charging energy costs and any maintenance.

For the generator, each kWh delivered during an outage costs $0.42 in fuel, plus an allocation of the $220/year maintenance over the energy produced each year. With 162 kWh of annual outage demand, maintenance alone adds about $220 ÷ 162 ≈ $1.36/kWh, bringing total cost well above $1.50/kWh before considering any capital cost for the generator itself. This illustrates how, under these assumptions, fuel and maintenance dominate generator costs, while batteries and EVs are more capital intensive but potentially cheaper per kWh over time.

Comparison at a Glance

Assumptions & Limitations

To keep the model transparent and easy to use, the estimator relies on several simplifying assumptions. These help you compare options consistently but also mean the results are illustrative rather than exact forecasts.

• EV battery cost is treated as sunk: the tool assumes you already own (or plan to own) the EV primarily for transportation. Only incremental hardware for V2H (such as a bidirectional charger) is treated as a backup capital cost.
• Bidirectional charger cost is approximated: in the underlying script, charger capital may be modeled as a fixed fraction of a standalone battery system cost to represent comparable inverter hardware. Actual pricing can differ by manufacturer and installation complexity.
• Outage support is energy-limited: the EV and standalone battery can only supply energy equal to their usable capacity multiplied by round-trip efficiency. If your outage demand exceeds that amount, the tool does not automatically layer in additional resources or load shedding.
• Generator costs are simplified: generator operating costs are based on your entered fuel cost per delivered kWh and annual maintenance only. The model does not explicitly include generator purchase price, installation, permitting, or replacement intervals unless the script adds them separately.
• Battery degradation is not fully modeled: while cycle life is considered for the standalone battery, the tool does not simulate detailed degradation curves, warranty conditions, state-of-health impacts, or EV resale value tied to additional cycling.
• Tariff and policy details are ignored: the estimator does not model time-of-use electricity rates, demand charges, export compensation (e.g., net metering), demand response incentives, or tax credits. All dollar values are treated in nominal terms without inflation adjustment.
• Discount rate usage: the discount rate you enter is used to annualize capital investments. It does not adjust fuel, electricity, or maintenance costs over time.
• Operational behavior is simplified: the model assumes that, when an outage occurs, your backup system operates at the specified average load for the specified outage hours. It does not capture dynamic load shedding, partial backup of critical circuits only, or complex dispatch strategies.

These assumptions mean the tool is best used for high-level planning and relative comparisons rather than detailed engineering or financial design. If you are making a major investment, you may want to refine these numbers with a professional who can account for local rates, codes, and site-specific factors.

This estimator is for educational and planning purposes only and does not constitute financial, engineering, or legal advice.

Who This Tool Is For

The calculator is intended for homeowners, energy consultants, and resilience planners who want a quick way to compare the economic implications of using EVs, home batteries, and generators for backup power. It can also support early-stage feasibility studies, program design, and conversations with installers or equipment vendors.

Methodology and default ranges are informed by typical values for modern EVs, residential batteries, and small standby generators as of the mid-2020s. For critical facilities or large commercial sites, more detailed modeling is recommended.